Write a PHP program that accepts user input for a student's first name, last name, mark, and module, and then writes this data to a text file. The program should:

$1.$ Accept input from the user via an HTML form, which includes fields for the following:

o$$ First Name

o$$ Last Name

o$$ Mark $($numeric input$)$

o$$ Module

$2.$ Validate the input to ensure that all fields are filled in before processing.

$3.$ Write the input data to a text file $($students$.$txt$),$ appending new entries each time the form is submitted. The data should be formatted as follows:

$4.$ Display an appropriate success message if the data is written to the file, or an error message if the writing process fails.

$$

PHP Code Structure: You should implement the following PHP logic:

$$ A form for input collection.

$$ Validation to ensure that all fields are filled.

$$ Code that writes the validated input to a text file.